1. Field of the Invention
This invention relates in general to drill string members used in the rotary drilling art of the oil and gas industry. In particular the invention relates to an intermediate weight member adapted for placement between relatively lighter weight drill pipe and heavier weight drill collars.
2. Description of Prior art
Intermediate weight members have been used in the art of rotary drilling as a transition member between stiff drilling collars and standard drill pipe. Such intermediate weight members reduce fatigue in the stress-susceptible transition zone between drill pipe and drilling collars.
A prior art intermediate weight member sold by OMSCO Industries, the assignee of the invention described herein, includes a relatively heavy weight tubular member having upper and lower sections separated by a short center wear pad. The member has a constant inner diameter along its entire length. The upper and lower sections are of constant outer diameter. The wear pad has an outer diameter which is slightly less than about one-half inch larger than the outer diameter of the upper and lower sections. The outer diameter of the upper section is small enough to enable handling by standard slips and elevators of a drilling rig. The cross-section moment of inertia of the OMSCO intermediate weight member is significantly greater at the center wear pad from that at the upper and lower constant outer diameter sections. Accordingly, the OMSCO intermediate weight member, because of its construction, can be characterized by upper and lower relatively limber sections separated by a relatively stiff center section.
Another intermediate weight drill string member is described in U.S. pat. no. 4,460,202 to Chance et.al. The Chance drill string member includes an upper relatively short slip and elevator section and a lower grooved section which extends substantially the remaining length of the member. The grooved section is of an outer diameter which is larger than the outer diameter of the slip and elevator section, but has spiral grooves formed therein of a number and depth such that the bending moment of inertia along the entire length of the member is substantially constant. In other words, the cross-section moment of inertia anywhere along the spiral section is substantially equal to the cross-section moment of inertia in the slip and elevator area. As a consequence, the Chance member can be characterized as having a single bending pattern which is substantially constant from one end of the member to the other. In addition, the spiralled grooved section provides advantages, like other spiralled members, of reduced potential for differential sticking in the hole, aiding in the removal of drilling chips up the annulus between the drill string and the borehole, and possible forward thrusting effect during drilling.
3. Identification of objects of the invention
It is a primary object of the invention to provide certain advantages of the two prior art intermediate weight members described above with a unique mechanical structure.
It is another object of the invention to provide an intermediate weight member which includes not only a slip and elevator section and spiralled sections, but may also include a center wear pad section.
It is still another object of the invention to provide a drill string member of intermediate weight having a structure such that the cross-section moment of inertia of the member gradually changes a significant amount as a function of length along the member so as to reduce bending stresses inherent in regions where bending moments abruptly change.